Ngo Duc Thanh, director of Tra Vinh animal health department, said the kinds of vaccine that have been used for many years showed weaker effects this year.

Thanh said most samples from sick poultry would be tested for a new mutation.

He asked farmers to inject their livestock with a new kind of vaccine that is also available.

Animal health officials from the central province of Quang Nam last week noticed that a local herd of 1,000 ducks have fallen sick and five of six samples tested positive for H5N1 although they were vaccinated a month earlier.

Vaccinated poultry from the Mekong Delta city of Can Tho were also infected earlier despite vaccination. Officials said the vaccine could have been ineffective to a virus mutation, or the vaccination process faulty.

The health and agriculture ministries on March 5 said there were 69 H5N1 bird flu outbreaks ravaging in 23 cities and provinces. An average of two new outbreaks have occurred every day since early February, killing more than 63,000 fowls.

According to health ministry regulations, a city/province can declare the end of a bird flu epidemic if no infections are detected 21 days after the last outbreak was announced, removing Nam Dinh Province outside Hanoi and the Mekong Delta’s Long An out of the list.

Hai Duong Province outside Hanoi, Gia Lai in the Central Highlands and the Mekong Delta’s Soc Trang Province announced new outbreaks on February 28, with more than 5,000 fowls disposed of.

Experts have warned of further spread of the virus thanks to cold weather.

Control teams have been set up by the health ministry and local authorities to keep track on the infection.

H5N1 has killed three people in the country this year, including Huynh Thanh Tuan, 30, of Nha Trang, a 52-year-old man from Binh Phuoc Province and a 60-year-old woman from Dong Thap Province, both in the south.

The strain has been ravaging Vietnam since 2003 and has claimed 65 lives so far, one of the highest fatality rates in the world, according to the World Health Organization.